In order to understand how microorganisms or their products modulate or deregulate immune systems, we have constructed monoclonal lines of immunocytes as cloned hybridomas. These express the phenotypes of various fractionated splenocytes. Hybridomas, representing mature and precursor regulatory T-cells, have been constructed from NFR/N mice and from NFR-nude mice respectively. Cytotoxic T-cells (CTL) have been cloned from 5-day splenocyte cultures in the presence of T-cell growth factors and antigen. We have used the toxins, streptococcal pyrogenic exotoxin (SPE) and pertussis toxin (PT) in parallel to study their immunomodulatory activities in vitro. We have shown that SPE interferes with the normal down regulation of T-cell dependent antibody responses. In experiments employing mature-type and precursor-type monoclonal T-cell clones, we observe that the cell generating the SPE effect is a precursor, rather than a mature functional T-cell. Whereas, the mature-type suppressor clones are antigen (carrier)-specific, the precursor-type clones are non-specific, non-NK, suppressor clones. One of these clones expressed the T-cell differentiation markers L3T4 and Lyt2 simultaneously in flow cytofluorometry. A subclone has been derived from growth in the presence of SPE with reduced suppressive activity and reduced expression of Lyt2. The change appears to be a sustained, phenotypic change through several generations. We have been unable, so far, to ascertain rearrangements in the alpha or beta chain gene of the T-cell receptor in either of these nude mouse T-cell hybridomas. SPE and PT are inhibitory to CTL responses; PT is about 10-fold more inhibitory than SPE. SPE had been shown to be only transiently inhibitory in PFC assays. Both toxins are active without cytotoxic effects. They are mitogenic at low concentrations. PT generates a cell-associated CTL suppressive activity in splenocyte cultures and an increased number of L3T4(-), Lyt2+, Thylt T-cells.